1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device based on silicon carbide.
2. Background Art
MOS semiconductor devices based on silicon materials are widely used for power switching and high-frequency switching applications. Such semiconductor devices for switching applications particularly require a high insulation breakdown voltage, low on-resistance, and fast switching rate. As compared with silicon, silicon carbide (SiC) has about ten times higher breakdown electric field and about three times higher thermal conductivity. For this reason, silicon carbide has the great potential as a material for fast and low-loss switching elements operable in high-temperature environments.
Furthermore, in silicon carbide, a silicon dioxide (SiO2) gate oxide film can be formed by thermal oxidation. Therefore silicon carbide is suitable for MOS semiconductor devices as with silicon (e.g., “1.4 kV 4H-SIC UMOSFET with Low Specific On-Resistance”, Proc. of 1998 ISPSD, pp. 119-122, 1998). There is a technical disclosure of a MOSFET having a trench configuration with reduced on-resistance (JP 2003-318409A).
In a MOSFET based on silicon carbide, a phenomenon is observed in which the electron mobility in the inversion layer formed at the gate oxide film interface of the base layer is lower than in the case of silicon. For example, the electron mobility is about 500 cm2/V·sec in silicon, but about 10 to 50 cm2/V·sec in silicon carbide. It is believed that this is attributed to trapping at the oxide film interface on the silicon carbide surface. In JP 2003-318409A mentioned above, an n−-type layer is provided between the gate oxide film and the p-type base layer to accumulate electrons, thereby improving the electron mobility.
In such a structure, a low on-resistance can be achieved, but the gate threshold voltage is as low as 1 to 2 volts, which is unsuitable for power semiconductor devices. That is, an extremely low gate threshold voltage undesirably causes the MOSFET to malfunction due to noise applied to the gate electrode.